The effects of PBB on the immune response were further evaluated following low level chronic and pre/postnatal exposure in mice and rats. In chronic exposure studies, rats were found to be more immune suppressed than mice (a 10X). T-cell functions were more suppressed than B-cell functions, although B-cell functions were effected at the highest dosage group as indicated by a decrease in functional immune tests as well as decreased resistance to infection with Listeria monocytogenes. No gram negative endotoxin hypersensitivity was found. Pre/postnatal exposure failed to induce any immune alteration although an increase in the number of colony forming units in the spleen and increased endotoxin hypersensitivity were noted.